English ‘dinosaur’ comes from a Greek word and it means 'terrible big lizard'.
The most important feature which you can distinguish dinosaur is their hind legs.
In their femur, there is a ball-shaped projection and it fits in a big hole(socket) in their pelvis.
Also, their ankle joints move only front-back direction, which is a sound structure.
Such features are related to the standing posture of dinosaurs and scientists give a definition of dinosaurs by these features of legs and other skeletons.
Elasmosaur which lived in the Mesozoic sea and flying pterosaur are not dinosaurs.
From the website of Fukui Prefectural Dinosaur Museum.